The liquid crystal display device is widely used as a variety of display means incorporated in an OA (office automation) device such as a word processor, personal computer or the like, and as a television set, car navigation monitor, aircraft-cockpit monitor or the like because of being advantageous in that it can be connected directly to an IC because of the operability with a low voltage and small power consumption, has many displaying functions, and can easily be designed lightweight and compact.
The liquid crystal display device uses the polarization plates to visualize a change of liquid crystal alignment. The polarization plate normally includes a polarizer and transparent protective films made of triacetyl cellulose (TAC) laminated over both sides of the polarizer. The polarizer is an optical element that splits incident light into two polarized components orthogonal to each other, and passes only one of the two polarized components (component which oscillates in a direction parallel with the transmission axis of the polarizer) while absorbing or dispersing the other component (component which oscillates in a direction parallel with the absorption axis of the polarizer).
The transmission type liquid crystal display device includes a liquid crystal cell which is sandwiched between polarization plates in its direction of thickness. The polarizers included in the opposite polarization plates, respectively, are normally disposed with their transmission axes being orthogonal to each other. The pair of polarizers whose transmission axes are orthogonal to each other is called “orthogonal polarizer”. Generally, the characteristic of the polarizer depends upon the viewing angle. Namely, when light is incident obliquely upon the polarizer, the transmission axis varies in direction. Therefore, even with the two polarizers being superposed one on the other for their transmission axes to be orthogonal to perpendicular incident light, the crossed-axes angle in relation to oblique incident light will not be the right angle, so that polarized light having passed through the first polarizer will have components parallel with the transmission axis of the second polarizer and which will pass through the second polarizer, resulting in light leakage.
The above dependence of the polarizer upon the viewing angle will narrow the range of viewing angle in which the brightness, contrast, tint and the like of an image on the screen of the liquid crystal display device are visually recognizable. To implement a liquid crystal display device whose viewing angle is larger, it is essential to develop a polarization plate having the range of viewing angle thereof widened for little light leakage by reducing the dependence of the polarizer upon the viewing angle, that is, a polarization plate assuring a wider viewing angle. Some polarization plates have been proposed heretofore.
In the Japanese Patent Application Laid Open No. 2001-350022, for example, it is proposed to use a biaxial retardation film for optical compensation of the polarizer in order to implement a liquid crystal display apparatus having the range of viewing angle thereof widened by reducing the dependence of the polarizer upon the viewing angle.
Note here that a so-called in-plane switching (IPS) mode type liquid crystal display device in which liquid crystal molecules are activated by a field parallel with a substrate is promising as an image display device which will substitute for CRT (cathode ray tube) because it is free from any image gradation inversion and color change when the viewing angle is large.
Even in the in-plane switching (IPS) mode type liquid crystal display device, however, light transmission takes place when the display screen is viewed at an angle at which it would have the black level, particularly, when the display screen is viewed at an angle of 45 deg. from the polarization axes of the pair of polarization plates forming together the display device, resulting in a lower contrast of the display screen.
The polarization plate is formed from a polarizer that passes predetermined-directional light and a protective layer laminated on either side of the polarizer. Normally, the protective layer is made of TAC or the like and the polarizer is sandwiched between the protective layers in the direction of thickness. Therefore, incident light having passed through the polarizer will pass through the protective layer interposed between the polarizer and liquid crystal cell, and arrive at a liquid crystal layer.
FIG. 1 shows a viewing angle when the display screen will have the black level. As the testing conditions for this viewing angle for a display at the black level, the absorption axis of the lower polarization plate (at the side of the back light) is set at an angle of 90 deg., absorption axis of the upper polarization plate is to an angle of 0 deg., liquid crystal is aligned at an azimuth of 90 deg. (in which the alignment direction is open through 90 deg. on a lower substrate), and the substrate is pretilted at an angle of 2 deg. The light path length difference ΔndLC of the liquid crystal layer is set to 275 nm (when light used has a wavelength of 550 nm). The protective layer included in the polarization plate is made of TAC and functions as a uniaxial retardation film having an optical axis perpendicular to the substrate. The light path length difference ΔndTAC0 of each protective layer interposed between the polarizers in the upper and lower polarization plates and the liquid crystal layer is 50 nm. In FIG. 1, each of letters a, to e shows an isoluminance contour plotted by a value of the ratio between incident light and leakage light, also shown. As will be seen, a large light transmission takes place in a direction of 45 deg.
FIG. 2 shows spectral transmittances at the time of a black-level display in the conventional liquid crystal display device when the display screen is viewed at an angle Pola (elevation angle) of 70 deg. and each of azimuths Azim of 15, 30, 45, 60 and 75 deg., and FIG. 3 shows spectral transmittances at the time of a black-level display in the conventional liquid crystal display device when the display screen is viewed at an angle Pola (elevation angle) of 70 deg. and each of azimuths Azim of 345, 330, 315, 300 and 285 deg. Note here that the azimuth Azim is a counterclockwise angle from a viewing angle of 0 deg. from the right in the drawings.
When the display screen is viewed at each of viewing angles are 45, 135, 225 and 315 deg. in a direction of 45 deg. from the polarization axis of the polarization plate, the light transmittance is high and the spectral transmittance provides a yellowish display, resulting in a lower quality of display.
The aforementioned Japanese Unexamined Patent Application Publication discloses the use of a biaxial retardation film for optical compensation of the polarizers, but not any interposition of a protective layer having a negative phase difference between the polarizer in the polarization plate and the liquid crystal layer. It does not disclose the use of any in-plane switching mode liquid crystal layer with the protective layer interposed as above. Therefore, even with the use of the technology disclosed in the Japanese Unexamined Patent Application Publication, it is difficult to overcome the problem of the lower display quality and compensate the dependence of the polarizer upon the viewing angle.